1. Field of Invention
This invention relates to electrical contactors and more particularly to electrical contactors in which the contacts are closed by controlling the application of voltage pulses to the coil of an electromagnet.
2. Background Information
Electrical contactors are electrically operated switches used for controlling motors and other types of electrical loads. An example of such an electrical contactor is disclosed in U.S. Pat. No. 4,720,763. These contactors include a set of movable electrical contacts which are brought into contact with a set of fixed contacts to close the contactor. The contacts are biased open by a kickout spring. A second spring, called a contactor spring, begins to compress as the moving contacts first contact the fixed contacts. The contactor spring determines the amount of current that can be carried by the contactor and the amount of contact wear that can be tolerated. The movable contacts are carried by the armature of an electromagnet. Energization of the electromagnet overcomes the spring forces and closes the contacts.
In earlier contactors, the energy applied to the coil of the electromagnet was substantially in excess of that required to effect closure. While it is desirable to have a positive closing to preclude welding of the contacts, the excess energy is unnecessary and even harmful. If the armature of the electromagnet seats while traveling at a high velocity, the excess kinetic energy is absorbed by the mechanical system as shock, noise, heat, vibration and contact bounce.
Pat. No. 4,720,763 discloses a contactor controlled by a microcomputer which triggers a track to gate full wave rectified ac voltage pulses to the electromagnet coil to more closely control the electrical energy used to close the contacts. The profile is divided into four phases: an acceleration phase; a coast phase; a grab phase; and a hold phase. In the acceleration phase, sufficient electrical energy is supplied to accelerate the armature to a velocity which gives the system enough kinetic energy to fully close the contacts against the spring forces. To assure positive closure, the kinetic energy imparted to the armature is such that it still has a small velocity as the armature seats against the magnet, but the excess energy is very small compared to that remaining at full closure in earlier contactors. The conduction angle of the track is selected to provide the previously empirically determined amount of energy needed during the acceleration phase.
In the exemplary system of Pat. No. 4,720,763, portions of two half cycles of the fullwave rectified voltage are gated to the electromagnet coil during the acceleration phase. The conduction angles for these two half cycles are stored in the microcomputer memory. In the coast phase, the armature loses velocity as the kickout spring is compressed and then decelerates more rapidly as the contacts touch and the heavier contactor spring begins to compress. A longer delay, and therefore, a smaller conduction angle is used for the one pulse provided during the coast phase. In the grab phase, the armature seats against the electromagnet. Three larger pulses, that is pulses with larger conduction angles, are used to seal the contacts in during the grab phase and prevent contact bounce. Ideally, the conduction angle for the grab phase is selected such that the first grab pulse is turned on just as the armature touches. In the hold phase, smaller pulses, that is pulses which are substantially phase delayed, are used to maintain contact closure.
In the acceleration grab and hold phases, feed forward control is used. Fixed values of the track conduction angle for these three phases are stored in computer memory. To accommodate for variations in the amplitude of the voltage pulses, Pat. No. 4,720,763 stores three values for each conduction angle for the acceleration, coast and grab phases for three ranges of the voltage amplitude. In the hold phase, a closed loop control circuit is used to maintain a coil current selected to maintain contact closure.
While the microcomputer controlled contactor of Pat. No. 4,720,763 is a great improvement over earlier contactors, and goes a long way toward controlling coil current during closure to reduce the kinetic energy of the armature as it seats against the electromagnet, there is room for improvement. For instance, it has been determined that the contact closure characteristic is dependent upon variations in coil resistance which are not taken into account by the control system of Pat. No. 4,720,763. Such changes in coil resistance are attributable to such factors as, for example, temperature changes and variations in the production process such as stretched wire. Thus, while a good closing sequence using a specific number of phased back half line voltage pulses was determinable experimentally, after a number of operations the profile required adjustment because the closing characteristics, such as contact bounce degraded. One difficulty in making adjustments in the closing profile is the very short duration of the entire cycle.
There is need therefore, for an improved contactor which provides positive closure without contact bounce.
There is also a need for such an improved contactor which uses phase controlled voltage pulses to provide the energy required for such positive closure without contact bounce.
There is an additional need for such a contactor which takes into account dynamic changes in the characteristics of the contactor electromagnet.
There is a further need for such a contactor which can make adjustments within the very short time frame of the closing sequence.